Technical Field
The present invention relates to a device in a silencer unit. More particularly, the invention relates to a device for use with a silencer unit in an exhaust system of an automotive vehicle for suppressing unwanted noise originating from an exhaust gas flow through said exhaust system.
Background Information
An exhaust system is used for discharging an exhaust gas flow from a vehicle engine to the surroundings. Here, the term xe2x80x9cexhaust systemxe2x80x9d is used to designate an arrangement comprising a number of tubular components that convey exhausts out of an exhaust manifold of the engine, a silencer unit, and, typically, a three-way catalytic converter. The silencer unit reduces unwanted noise originating from the exhaust gas flow through the exhaust system, whereas the three-way catalytic converter reduces the emission of harmful contaminants from the engine.
The above silencer unit functions evening out pulsation of the flowing exhaust gases, and making this pulsation as inaudible as possible. In order to enhance the comfort of the vehicle passengers, today""s silencer units are designed to reduce noise from the exhaust gas flow to a level below a predetermined limit value. The xe2x80x9climit valuexe2x80x9d designates a varying value (or a set of values) depending on the frequency of the noise. Due to statutory requirements in various countries regarding the noise level that can be perceived outside the vehicle, i.e., that influences the vehicle""s exterior environment, it is necessary to limit the noise level in order to meet these requirements.
Consequently, the exhaust system has to be designed in such a way as to meet both the comfort requirements as well as the statutory requirements with regard to acceptable noise levels.
Known silencer units are arranged as a termination of the exhaust system and comprises a chamber through which the exhaust gases are conveyed. The outlet from this chamber is a tubular end pipe through which exhaust gases flow. From the end pipe, the exhaust gases are passed into a further tubular member positioned in a conventional manner so as to extend from the tail end of the vehicle.
Present silencer units are preferably filled, completely or in part, with mineral wool or a similar sound-absorbing material. Furthermore, the end pipe may be perforated, i.e., provided with a number of relatively small holes, distributed along the circumferential surface of the end pipe. This allows the high-frequency energy in the exhaust gas flow to vent out to the surroundings and be absorbed by the sound-absorbing material, which preferably encompasses the end pipe perforations. This results in a reduction of the sound level from the exhaust gas flow, mainly at relatively high frequencies. A reduction of the noise at relatively low frequencies is achieved through a suitable sizing of the length and diameter of the end pipe.
The sound-absorbing properties of a silencer unit in a vehicle are determined by several factors, e.g., the type of engine, the flow properties of the exhaust gases, and the design and the dimensions of the exhaust system. Not least, the sound-absorbing properties are influenced to a high degree by the configuration of the passenger compartment of the vehicle in question. In some vehicles, the geometry and dimensions of the passenger compartment contribute to a relatively high noise level at relatively low frequencies, particularly in the order of about 50 to about 70 Hz, due to stationary wave conditions and resonance inside the compartment. This type of low-frequency noise is very unfavorable from a comfort aspect and constitutes a problem when exceeding acceptable noise level limits.
The problem of excessively loud low-frequency noise may be resolved by utilizing a relatively long end pipe that is not equipped with perforations. This would then mean that the low-frequency noise could be reduced to a level that would be acceptable from a comfort aspect, while simultaneously having the effect of substantially reducing the absorption of high-frequency noise. There would then arise a risk of the noise absorption in relatively high frequencies, particularly in the order of about 150 to about 200 Hz, being unsatisfactory. In worst cases, this could lead to not being able to meet statutory requirements regarding exterior, high-frequency noise from the vehicle.
Consequently, there are contradictory requirements regarding the object of striving to suppress the low-frequency noise while at the same time meeting statutory requirements regarding the levels of the exterior high-frequency noise. An exhaust system having an increased volume might solve this problem. However, this is not always possible, as the exhaust system has to be sized and designed to meet those requirements regarding its mounting space in the vehicle, as well as its cost, weight and similar factors.
U.S. Pat. No. 4,673,058 teaches an automotive muffler having a low frequency tuning chamber and a high frequency tuning chamber, the latter being packed with a sound absorbing material.
U.S. Pat. No. 5,025,890 teaches an engine exhaust apparatus having two outlet pipes, and which is arranged so as to lower the level of high frequency noise and also create non-offensive, comfortable and sporty sounds, especially in a low or intermediate frequency region.
Furthermore, U.S. Pat. No. 3,393,160 teaches a silencer provided with an outlet tube having a series of orifices.
The present invention provides an improved device for a silencer unit of an automotive vehicle, which, for reasons of comfort, provides a high degree of damping of low-frequency noise while at the same time, in response to existing statutory requirements, provides a high absorption of high-frequency noise.
The invention constitutes a device for a silencer unit of an automotive vehicle for suppressing unwanted noise originating from an exhaust gas flow from the vehicle engine. According to the invention, the silencer unit is terminated by an end pipe through which exhaust gas flow is conveyed to the surroundings. The end pipe has at least one perforated section with a predetermined extension in the longitudinal direction of the end pipe positioned downstream of a point along the end pipe where there will be substantially no turbulence in the exhaust flow. Through the invention, a high suppression of low-frequency noise as well as a good absorption of high-frequency noise is achieved. A further advantage of the invention is that it provides a passive silencing system that is simple, robust and cost-effective. According to the prior art, there is generally a maximum allowable length for an end pipe. If the end pipe is made too long (which might be required for suppressing low-frequency noise), there is a risk of resonance occurring. This may lead to audible tones being generated, which would be undesirable. However, according to the present invention this maximum length can be exceeded without creating unwanted tones, which is a further advantage of the invention.
Other advantageous embodiments are disclosed herein below.